Biological Conservation 98 (2001) 241±249 www.elsevier.com/locate/biocon Evidence for spider community resilience to invasion by non-native spiders Jutta C. Burger a,*, Michael A. Patten b, Thomas R. Prentice a, Richard A. Redak c aDepartment of Entomology, University of California, Riverside, CA 92521, USA bDepartment of Biology, University of California, Riverside, CA 92521, USA cDepartment of Entomology and Center for Conservation Biology, University of California, Riverside, CA 92521, USA Received 14 June 2000; received in revised form 20 June 2000; accepted 17 August 2000 Abstract The negative impacts of non-native species are well documented; however, the ecological outcomes of invasions can vary widely. In order to determine the resilience of local communities to invasion by non-native spiders, we compared spider assemblages from areas with varying numbers of non-native spiders in California coastal sage scrub. Spiders were collected from pitfall traps over 2 years. Productive lowland coastal sites contained both the highest proportion of non-natives and the greatest number of spiders overall. We detected no negative associations between native and non-native spiders and therefore suggest that non-native spiders are not presently impacting local ground-dwelling spiders. Strong positive correlations between abundances of some natives and non-natives may be the result of similar habitat preferences or of facilitation between species. We propose that the eects of non- native species depend on resource availability and site productivity, which, in turn, aect community resilience. Our results support the contention that both invasibility and resilience are higher in diverse, highly linked communities with high resource availability rather than the classical view that species poor communities are more invasible. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Non-native species; Biological invasions; Invasibility; Community resilience; Arthropoda; Araneae 1. Introduction on population levels (but may or may not alter foraging behavior, habitat choice, etc.). That an entire ecosystem Non-native species frequently pose a threat to the can be aected, directly or indirectly, by an exotic spe- persistence of natives, presumably through competitive cies is exempli®ed by such classic cases as the brown tree exclusion or depredation. Several examples of the snake Boiga irregularis in Guam (Fritts and Rodda, devastating, community-wide eects of introduced 1998), the red imported ®re ant Solenopsis invicta in organisms exist (e.g., Enserink, 1999, Strayer, 1999), but Texas (Porter and Savignano, 1990), and cheatgrass these examples represent extreme cases. More com- Bromus tectorum in shrub-steppe vegetation (Brandt monly, we encounter non-native species seemingly and Rickard, 1994). The eects that non-native species coexisting with natives, and for the most part we are can have on populations of unrelated species have also unaware of their eects (Aniansson, 1999). been documented, as with European Starlings Sturnus Non-native organisms potentially exert four distinct vulgaris outcompeting nesting Northern Flickers Cola- types of eects on a native species or community. They petes auratus (Ingold, 1996) and Purple Martins Progne may (1) impact and degrade an entire community or subis (Brown, 1997). Reports of negative eects on ecosystem; (2) cause declines in speci®c, possibly unre- populations of related species are numerous. The non- lated, native species or small sets of unrelated species native cray®sh Orconectes rusticus outcompetes native (presumably through competition or predation); (3) Orconectes species in Wisconsin (Hill and Lodge, 1999), cause declines in closely related taxa (most likely bullfrogs Rana catesbiana deplete populations of native through competition); or (4) have no discernible eect Rana species throughout California (Hayes and Jen- nings, 1986), and Argentine ants, Linepithema humile, * Corresponding author. depress many native ant species' populations along E-mail address: [email protected] (J.C. Burger). urban interfaces in California (Holway, 1999, Suarez et 0006-3207/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(00)00159-2 242 J.C. Burger et al. / Biological Conservation 98 (2001) 241±249 al., 1998). Published cases of exotics not aecting natives that have become established in southern Cali- natives are less common (Bauchau, 1997), but do exist. fornia within the last 100 years (Chamberlin and Ivie, For example, non-native ®shes in some streams of 1935, Gertsch, 1979, Platnick and Shadab, 1983, Plat- northern California are not signi®cantly changing native nick and Murphy, 1984). Four of these seven are gna- species assemblages (Baltz and Moyle, 1993), in contrast phosids, the most abundant and diverse group of native to their eects in other regions of North America ground-dwelling spiders in CSS (Prentice et al., 1998). (Moyle, 1995, Chapleau et al., 1997). They represent a single guild (sensu Root, 1967) of It is thus dicult to predict the degree to which non- wandering, generalist predators. One, Dysdera crocata natives are able to invade ecosystems. Elton (1958), (Dysderidae), is an isopod specialist (Gertsch, 1979). MacArthur (1970), Stachowicz et al. (1999) and others Another, Oecobius annulipes (Oecobiidae), is a small, proposed that species-poor ecosystems are most easily relatively sessile web-builder. Yet another, Metaltella invaded. Such systems contain species depauperate simoni (Amphinectidae), is a moderately large ground communities that may re¯ect low resource availability dweller. or high levels of disturbance and result in a less ecient Based on generally accepted theory, if the hypothesis utilization of resource space. However, the bulk of of higher invasibility of low diversity sites is true, then experimental data suggest the opposite trend: areas of sites with lower species richness should have more non- high diversity are more likely to support invading spe- natives (and, by corollary, vice versa). Thus, our objec- cies (Levine and D'Antonio, 1999, Levin, 2000). Diver- tives were (1) to describe the pattern of occurrence of sity may re¯ect high resource availability, high non-native spiders across sites in CSS, (2) to determine productivity, and high structural complexity. the degree of association between native spiders, non- Similar disagreement exists as to which conditions native spiders, and their environment, and (3) to study most favor large scale eects on resident fauna (Levine the intraguild relationships between closely related and D'Antonio, 1999). Whatever the optimal conditions native and exotic gnaphosid spiders. We predicted that, for invasion and establishment may be, it is certain that if non-natives were impacting native spider commu- habitat fragmentation and anthropogenic in¯uences nities, their eects would be most obvious within the increase both the likelihood of invasion and the severity ecologically and behaviorally similar gnaphosids. of the invaders' eects on natives (Enserink, 1999). We have studied geographic patterns in local spider assem- blages in order to determine which of these processes 2. Methods may apply to ground-dwelling spider communities in southern Californian coastal sage scrub (CSS) habitats. 2.1. Field collection and identi®cation CSS is a major vegetation type in cismontane south- ern California (Westman, 1981). Its canopy is composed In order to describe patterns of abundances of native of drought-deciduous shrubs such as Artemisia cali- and non-native spiders, we sampled spiders from 60 fornica (Asteraceae) and Eriogonum fasciculatum (Poly- sites in undisturbed CSS and chaparral intergrades gonaceae). Its understory was historically composed of across coastal San Diego County, California, USA. a rich array of native annual forbs. Primarily as a result Sites were located at Marine Corps Base Camp Pendle- of urban development, associated changes in ®re fre- ton and Marine Corps Air Station Miramar (Prentice et quency, and increased pollution, CSS has become al., 1998). Spiders were collected during 1-month sam- increasingly fragmented and vulnerable to invasion by pling periods in December 1994 and 1995, May 1995 non-native plant and animal species (Westman, 1979, and 1996, and August 1995 from ®ve randomly placed Zedler et al., 1983, Stylinski and Allen, 1999). Both 473 ml (16 oz.) pitfall traps at each site. Traps remained non-native grasses (e.g. Bromus spp., Avena spp.) in place for the duration of the study. Collection cups and annual forbs (e.g. Erodium spp., Centaurea sol- contained a dilute aqueous solution of AlconoxTM stitialis) are now common in most CSS and are detergent and salt to increase miscibility and decrease threatening native species' persistence (Eliason and decay. Pitfall traps have been shown to be eective Allen, 1997). To our knowledge, only one invasive when estimating cursorial spider (and other arthropod) arthropod species, the Argentine ant, has been studied fauna richness and activity (Uetz and Unzicker, 1976) in CSS. This species outcompetes and eliminates native and are currently the most accepted method for con- ants in CSS within valleys along urban edges (Suarez ducting ecological studies on these species. Traps were et al., 1998). left open for 7 days during each collection period. All CSS contains a diverse spider fauna whose species mature (and distinct immature) spiders collected were composition and response to non-native invasion